Paul Krugman & the nature of economics: Paul Krugman is being accused of hypocrisy for calling for an extension of unemployment benefits when one of his textbooks says "Generous unemployment benefits can increase both structural and frictional unemployment." I think he can be rescued from this charge, if we recognize that economics is not like (some conceptions of) the natural sciences, in that its theories are not universally applicable but rather of only local and temporal validity.
What I mean is that "textbook Krugman" is right in normal times when aggregate demand is highish. In such circumstances, giving people an incentive to find work through lower unemployment benefits can reduce frictional unemployment (the coexistence of vacancies and joblessness) and so increase output and reduce inflation.
But these might well not be normal times. It could well be be that demand for labour is unusually weak; low wage inflation and employment-population ratios suggest as much. In this world, the priority is not so much to reduce frictional unemployment as to reduce "Keynesian unemployment". And increased unemployment benefits - insofar as they are a fiscal expansion - might do this. When "columnist Krugman" says that "enhanced [unemployment insurance] actually creates jobs when the economy is depressed", the emphasis must be upon the last five words.
Indeed, incentivizing people to find work when it is not (so much) available might be worse than pointless. Cutting unemployment benefits might incentivize people to turn to crime rather than legitimate work.
So, it could be that "columnist Krugman" and "textbook Krugman" are both right, but they are describing different states of the world - and different facts require different models...